Digital cameras comprise semiconductor cells, such as CCD (Charge Coupled Device) or CMOS (Complementary Metal-Oxide Semiconductor) cells comprising a photosensitive sensor. The operation of the sensor in the cell is based on the principle that it is always charged when radiation impinges on it. The cell comprises densely placed parallel pixels which convert light to electrical signals. For colour detection, the pixels of the cell are covered by a filter which transmits, at certain pixels, only red colour, only green colour and only blue colour, respectively, forming a colour filter array. There is a given number (N) of bits per each pixel. If the data processing unit is capable of processing images of M bits and if N>M, a conversion of the image to a smaller size is needed, which means a reduction in the number of bits per pixel.
Normally, the number of bits per pixel is 8 bits, but there are cases in which a greater number is used, for example 10 bits per pixel. Normally, in a data processing device, one byte consists of 8 bits; therefore, it will be necessary to compress an image of 10 bits.
Furthermore, the compression of the image is necessary, because the digital reproduction and processing of images is no longer limited to computers but, to an increasing extent, images can be produced and displayed with smaller devices, such as, for example, mobile communication devices. In mobile communication devices, the use of images is largely similar to the use in computers. Images are stored in the device and they are transmitted to another device across a communication network in use. The transmission of images in the communication network, such as a mobile telephone network, is problematic because of the large quantity of information. Because the available communication channel is slow, the image material must be compressed so that several minutes would not be taken for the transmission of a single image.
DPCM (differential pulse code modulation) is a known method, by which a pixel is encoded/transmitted on the basis of the preceding pixel. The method is used for the conversion of an analog signal to a digital signal, wherein the difference between a sampled value of the analog signal and its predicted value is quantized and encoded in digital format. Code words formed by the DPCM method represent differences between values.
By differentiating the pulse code modulated (PCM) code words, a DPCM code word sequence of variable length is obtained, which normally compresses the given data to a format suitable for the transmission. Because the code words are normally of variable length, the result is not always necessarily below the predetermined bit number (M), because the longest code word is longer than the original number (N) of bits. To make sure that the code word is shorter than the limit value (M), the DPCM code words must be quantized, after which the compression method becomes lossy.
This problem is discussed in the publication DE 4210246 A1 which discloses a DPCM image codec supplemented with PCM technology. A questionable method is used to reduce the quantity of data in a digital image. In the method, the process of encoding digital data is used to convert the data with M bits per pixel to a code word of N bits, when N<M. The choice between the PCM and DPCM methods is made with respect to the difference values in the grey level. The decoding process is used to make sure that the length of the produced code word is less than 9 bits, but this code word length is not fixed but it varies according to the grey level changes in the image. The method presented in the publication is based on a state machine which will require control signals to operate. The decoding of the pixels of varying length is thus strongly dependent on other pixels. Another problem in the decoding of pixels with varying length according to related art is that it is not easy to determine from the code stream, which pixel each code represents.